Acceleration sensor and process for the production thereof

ABSTRACT

A single crystal silicon substrate ( 1 ) is bonded through an SiO 2  film ( 9 ) to a single crystal silicon substrate ( 8 ), and the single crystal silicon substrate ( 1 ) is made into a thin film. A cantilever ( 13 ) is formed on the single crystal silicon substrate ( 1 ), and the thickness of the cantilever ( 13 ) in a direction parallel to the surface of the single crystal silicon substrate ( 1 ) is made smaller than the thickness of the cantilever in the direction of the depth of the single crystal silicon substrate ( 1 ), and movable in a direction parallel to the substrate surface. In addition, the surface of the cantilever ( 13 ) and the part of the single crystal silicon substrate ( 1 ), opposing the cantilever ( 13 ), are, respectively, coated with an SiO 2  film ( 5 ), so that an electrode short circuit is prevented in a capacity-type sensor. In addition, a signal-processing circuit ( 10 ) is formed on the single crystal silicon substrate ( 1 ), so that signal processing is performed as the cantilever ( 13 ) moves.

This application is a Divisional Application of Ser. No. 10/123,220,filed Apr. 17, 2002 which is a Re-Issue Application of U.S. Pat. No.6,227,049.

Notice: More than one reissue application has been filed for reissue ofU.S. Pat. No. 6,227,049. The reissue applications are application Ser.Nos. 10/123,220, 10/315,566, 10/315,859, 10/315,827 (the presentapplication), 10/315,861, all of which are Divisional reissues of U.S.Pat. No. 6,227,049.

This is a continuation of application Ser. No. 08/167,976, filed on May11, 1994, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an acceleration sensor, and morespecifically, a semiconductor type acceleration sensor suitable for aair-bag system, a suspension control system, or the like, forautomobiles.

2. Description of the Related Art

In producing a semiconductor type acceleration sensor, the movable partthereof has hitherto been prepared in such a way that it penetrates asingle crystal silicon wafer. Accordingly, the movable part must besized so as to penetrate through the thickness of a wafer, andtherefore, it has been difficult to miniaturize the acceleration sensor.In addition, in order to incorporate this sensor into a package, arelaxation part, have been required such as a pedestal, in order torelease the stress caused by the difference in coefficients or thermalexpansion or the like.

However, in Nikkei Electronics, Nov. 11, 1991 (No. 540), pp 223 to 231,there is illustrated an acceleration sensor produced by use of a surfacemicro-machining technique. According to this technique, a thinpolysilicon film is laminated on a silicon substrate, and thispolysilicon film is etched, whereby a beam movable parallel to thesurface of the substrate is formed, so as to form a differentialcapacity type acceleration sensor. However, when a beam structure isformed by use of polysilicon, if a signal processing circuit is formedaround the formed beam structure, the sensor characteristics becomeunstable. This is because the beam structure is formed of apolycrystalline and amorphous material, resulting in noticeablevariation for every production lot. Accordingly, it is still desirableto form an acceleration sensor by surface micro-machining single crystalsilicon.

Under such circumstances, the purpose of the present invention is toprovide an acceleration sensor having a novel structure, by which highprecision and high reliability can be realized.

In addition, another purpose of the present invention is to produce thisacceleration sensor with a good yield rate during the production processthereof.

SUMMARY OF THE INVENTION

The gist of a first embodiment of the present invention resides in anacceleration sensor, comprising a second single crystal siliconsubstrate bonded onto a first single crystal silicon substrate with aninsulating film interposed therebetween, the second single crystalsilicon substrate being made of a thin film, a beam formed on at leasteither of said first and second single crystal silicon substrates andmovable in a direction parallel to the surface thereof, and asignal-processing circuit formed on at least one of said first andsecond single crystal silicon substrates for performing processing ofsignals produced by a movement of beam, caused by an acceleration.

In addition, the gist of a second embodiment of the present inventionresides in a process for producing an acceleration sensor, comprising; afirst step of forming, on a main surface of a first single crystalsilicon substrate, a groove with a predetermined depth for formation ofa beam; a second step of forming, on the main surface of said firstsingle crystal silicon substrate, a film of a polycrystaline silicon, anamorphous silicon or a mixture thereof so as to fill said groove withsaid silicon film, and smoothing (flattening) the surface of saidsilicon film; a third step of bonding the main surface of said firstsingle crystal silicon substrate to a second single crystal siliconsubstrate with an insulating film formed thereon, said insulating filmbeing interposed between said first and second single crystal siliconsubstrates; a fourth step of polishing the reverse side of said firstsingle crystal silicon substrate to a predetermined degree, so as tomake said first single crystal silicon substrate a thin film; and afifth step of forming a signal-processing circuit on at least either ofsaid first and second single crystal silicon substrates, and thereafter,removing by etching said silicon film of a polycrystal silicon, anamorphous silicon or a mixture thereof from said reverse side of saidfirst single crystal silicon substrates, to form a beam.

In addition, the gist of a third embodiment of the present inventionresides in a process for producing an acceleration sensor, comprising; afirst step of bonding a main surface of a first single crystal siliconsubstrate to a second single crystal silicon substrate with aninsulating film formed thereon, said insulating film being interposedtherebetween; a second step of polishing the reverse side of said firstsingle crystal silicon substrate to a predetermined degree, so as tomake the first single crystal silicon substrate a thin film; a thirdstep of forming a groove with a predetermined depth for formation of abeam; a fourth step of forming, on the reverse side of said first singlecrystal silicon substrate, a film of a polycrystal silicon, an amorphoussilicon or a mixture thereof, so as to fill said groove with saidsilicon film, and smoothing the surface of said silicon film; and afifth step of forming a signal-processing circuit on at least one ofsaid first and second single crystal silicon substrates, and thereafter,removing by etching said film of polycrystal silicon, amorphous siliconor a mixture thereof from the reverse side of the first single crystalsilicon substrate, to form a beam.

In the first embodiment, when an acceleration is applied in a directionparallel to the surface of the bonded single crystal silicon substrates,the beam formed on the first or second single crystal silicon substratemoves. As this beam moves, signal processing is performed in thesignal-processing circuit formed on the first or second single crystalsilicon substrate.

In the second embodiment, as a first step, a groove of a predetermineddepth for formation of a beam is formed on the main surface of the firstsingle crystal silicon substrate, and as a second step, a film of apolycrystalline silicon, an amorphous silicon or a mixture thereof isformed on the main surface of the first single crystal siliconsubstrate, whereby the groove is filled with the silicon film, and thesurface of this silicon film is flattened. Subsequently, as a thirdstep, the main surface of the first single crystal silicon substrate isbonded to a second single crystal silicon substrate having an insulatingfilm formed thereon, said insulating film being interposed between thefirst and second single crystal substrates, and, as a fourth step, thereverse side of the first single crystal silicon substrate is polishedto a predetermined degree, whereby the first single crystal siliconsubstrate is made into a thin film. Subsequently, as a fifth step, asignal-processing circuit is formed on the first or second singlecrystal silicon substrate, thereafter the polycrystalline, amorphous ormixed silicon film is removed by etching from the reverse side of thefirst single crystal silicon substrate, and a beam is formed. As aresult, an acceleration sensor according to the first invention isproduced.

In the third embodiment, as a first step, the main surface of a firstsingle crystal silicon substrate is bonded to a second single crystalsilicon substrate with an insulating film formed thereon, saidinsulating film being interposed between the first and secondsubstrates, and as a second step, the reverse side of the first singlecrystal silicon substrate is polished to a predetermined degree, so thatthe first single crystal silicon substrate is made into a thin film.Subsequently, as a third step, a groove of a predetermined depth forformation of a beam is formed on the reverse side of the first singlecrystal silicon substrate, and as a fourth step, a film ofpolycrystalline silicon, an amorphous silicon or a mixture thereof isformed on the reverse side of the first single crystal siliconsubstrate, whereupon the groove is filled with the silicon film, and thesurface of the silicon film is flattened. Subsequently, as a fifth step,a signal-processing circuit is formed on the first or second singlecrystal silicon substrate, whereafter the polycrystalline, amorphous ormixed silicon film is removed by etching from the reverse side of thefirst single crystal silicon substrate, and a beam is formed. As aresult, an acceleration sensor according to the first embodiment isproduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan of an acceleration sensor according to the firstembodiment of the present invention;

FIG. 2 is a cross section taken along line 2—2 in FIG. 1;

FIGS. 3 to 10 are illustrations for explanation of the manufacturingprocess of the first embodiment, and, respectively, a sectional view foreach production stage;

FIG. 11 is a plan illustrating an application example of the firstembodiment;

FIG. 12 is a sectional view taken along the line 12—12 in FIG. 11;

FIGS. 13 to 21 are illustrations for explanation of the manufacturingprocess of the second embodiment, and, respectively, a sectional viewfor each production stage;

FIGS. 22 to 28 are illustrations for explanation of the manufacturingprocess of the third embodiment, and, respectively, a sectional view foreach production stage;

FIGS. 29 to 31 are illustrations for explanation of the manufacturingprocess of the fourth embodiment, and, respectively, a sectional viewfor each production stage;

FIGS. 32 to 34 are illustrations for explanation of the manufacturingprocess examples to which the fourth embodiment is applied, and,respectively, a sectional view for each production stage;

FIG. 35 is a plan illustrating an example of a sensor tip formed by anacceleration sensor according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

In the following, the embodiments and realizations of the presentinvention, will be explained with reference to the attached drawings.

First Embodiment

FIG. 1 is a plain view plan of an acceleration sensor produced by thefirst embodiment of the present invention, and FIG. 2 is a sectionalview taken along the line 2—2 in FIG. 1. In addition, in the presentembodiment, the sensor part and the signal-processing circuit are formedof the same single crystal silicon substrate.

The present acceleration sensor is a capacity type acceleration sensor.As illustrated in FIG. 2, there is bonded onto a single crystal siliconsubstrate 8 a single crystal silicon substrate 1 through an SiO₂ film 9,and in the single crystal silicon substrate 1, there is formed acantilever 13 by a trench 3 penetrating said substrate 1. As illustratedin FIG. 1, this cantilever 13 has a structure such that an end sidethereof is branched into two parts. The cantilever 13 can be moved in adirection parallel to the surface of the single crystal siliconsubstrate 1 (the arrow direction C in FIG. 1). In addition, in thesingle crystal silicon substrate 1, a signal-processing circuit 10 isformed and is electrically insulated from the cantilever 13 by apolysilicon film 6 and an SiO₂ film 5.

In the following, the production precess of the first embodiment of thepresent invention, which is suitable for the production of the aforesaidstructure, will be explained with reference to FIGS. 3 to 10.

First, as illustrated in FIG. 3, an n-type (100) single crystal siliconsubstrate 1 with a resistance of 1 to 20Ω·cm is provided, and on themain surface thereof, there is formed an SiO₂ film 2 with a thickness ofabout 1 μm by thermal oxidation, and the SiO₂ film 2 is formed into apredetermined pattern by a photolithographic method. This pattern is apattern exposing, on the main surface of the silicon substrate, an areato be formed as a groove separating at least an area to be formed as amovable electrode (cantilever) from the substrate, and in the presentembodiment, this pattern is formed as a pattern exposing also the mainsurface of the silicon substrate in an area for transversely insulatingand separating the signal-processing circuit. Subsequently, on the mainsurface side of the single crystal silicon substrate 1, there is formeda trench 3 having a perpendicular wall with a predetermined depth, e.g.ranging from about 0.2 to about 30 μm. In the present embodiment,explanation will be made with respect to the case where the wallthickness is about 3 μm.

Subsequently, after the SiO₂ film 2 has been removed, as illustrated inFIG. 4, an n⁺ diffused layer 4 formed using phosphorus, arsenic or thelike is formed on the main surface of the single crystal siliconsubstrate 1, including the internal wall of the trench 3, and further,an SiO₂ film 5 with a thickness ranging from 0.1 to 1 μm is formed bythermal oxidation or the like. In this case, in order to prevent damageby etching, there may be conducted the so-called “sacrifice oxidation”in which SiO₂ is formed by thermal oxidation and removed beforeformation of the n⁺ diffused layer 4.

Subsequently, as illustrated in FIG. 5, a polysilicon film 6 is formedon the main surface of the single crystal silicon substrate 1, and thetrench 3 is filled with the polysilicon film 6. In addition, in the casewhere an impurity is introduced into the polysilicon film 6 in order touse the polysilicon film 6 as a conductive path, forming a thinpolysilicon layer and diffusing phosphorus or the like in a highconcentration before formation of the polysilicon film 6, allows theimpurity to be introduced into the polysilicon film 6.

Next, as illustrated in FIG. 6, the surface of the polysilicon film 6 ismirror finished, so as to make the thickness of the polysilicon film 6 apredetermined value. Subsequently, boron ions or other impurity areimplanted into the polysilicon film 6 to form p⁺ diffused layers 7 inpredetermined areas.

On the other hand, as illustrated in FIG. 7, another (100) singlecrystal silicon substrate 8 is provided, and an SiO₂ film 9 with athickness ranging from 0.1 to 1.0 μm is formed on the main surface ofthe silicon substrate 8.

Subsequently, the single crystal silicon substrate 1 and the singlecrystal silicon substrate 8 are dipped into e.g. an aqueous mixedsolution of aqueous hydrogen peroxide and sulfuric acid, so as tosubject these substrates to a hydrophilicily-imparting treatment.Thereafter, these substrates are dried, and as illustrated in FIG. 8,the main surface of the single crystal silicon substrate 1 and that ofthe single crystal silicon substrate 8 are put together at roomtemperature, and these substrates are introduced into a furnace at 400to 1100° C. for a period of time ranging from 0.5 to 2 hours, so as tostrongly bond them.

Subsequently, as illustrated in FIG. 9, by use of an aqueous solution ofan alkali type, e.g. an aqueous KOH solution or the like, the reverseside of the single crystal silicon substrate 1 is subjected to selectivepolishing. The polishing is continued until the SiO₂ film 5 is exposed.As a result, the thickness of the single crystal silicon substrate 1reaches a value which is approximately determined by the depth of thetrench, e.g. about 3 μm, so that the substrate 1 is made thin.

Subsequently, as illustrated in FIG. 10, in a predetermined area of thesingle crystal silicon substrate 1, there is formed a signal-processingcircuit 10 (IC circuit portion) by use of an ordinary CMOS process,bipolar process or the like. In addition, a diffused layer forconnecting to wiring and a metallic electrode film composed of analuminum film or the like are formed, and wiring at the sensor part,wiring at the circuit part, and connection of the sensor part to thecircuit part are performed. In FIG. 1 and FIG. 10, a MOS transistoralone is illustrated as a part of the signal-processing circuit 10.

Further, on the upper surface of the signal-processing circuit 10, thereis formed, as a passivation film 11, a plasma SiN film (P-SiN) e.g. by aplasma CD method. Subsequently, at the sensor part side, windows 12 areopened at predetermined areas of the passivation film 11, and thepolysilicon film 6 filled in the trench 3 is exposed from the surface.By this window opening procedure, the single crystal portions where acantilever or fixed electrodes are to be formed are demarcated from thepolycrystal portions embedded in the trench, on the surface of thesubstrate.

Subsequently, as illustrated in FIG. 2, by use of a 20% solution of TMAH(tetramethylammonium hydroxide) (CH₃)₄NOH, the polysilicon film 6 isremoved by etching through the windows 12 of the passivation film 11,from the reverse side (the upper side in FIG. 2) of the singlecrystalline silicon substrate 1. In this case, the passivation film 11(P-SiN), SiO₂ film 5, aluminum wiring layer and p⁺ diffused layer (p⁺polysilicon film) 7 are hardly etched by the selective etching.Accordingly, the bonding of the single crystal silicon substrate to thelower single crystal silicon substrate 8 is secured through the p⁺diffused layer (p⁺ polysilicon film) 7.

In addition, if trenches 3 are preliminarily formed also in the widearea of the cantilever 13 in FIG. 1, and etching windows 48 are providedsimultaneously with the formation of the windows 12, in such a way thatthe etching windows 48 communicate with said trenches 3, the polysiliconfilm 6 below the movable part (cantilever 13) of the sensor can be moresecurely removed by etching through this etching window 48 when thepolysilicon film 6 is removed by etching.

By the aforesaid procedures, the cantilever 13 is formed. In this case,the cantilever 13 becomes, as illustrated in FIG. 2, smaller in thethickness L2 in a direction parallel to the surface of the singlecrystal silicon substrate 1 than in the thickness L1 in a directionalong the depth of the single crystal silicon substrate 1.

In a capacity type acceleration sensor, the end portions (bifurcatedportions) of the cantilever 13 are formed as movable electrodes, and asillustrated in FIG. 1, the parts of the single crystal silicon substrate1, opposing the end portions of the cantilever 13, are formed as fixedelectrodes 14, 15, 16, and 17, respectively. In addition, as illustratedin FIG. 1, fixed electrode 14 and fixed electrode 16 are derived throughan aluminium wiring layer 18a, fixed electrode 15 and fixed electrode 17are derived through an aluminium wiring layer 18b, and the cantilever 13(movable electrode) is derived through an aluminium wiring layer 18c.These aluminium wiring layers 18a, 18b, and 18c are connected to thesignal-processing circuit 10, and by this signal-processing circuit 10,signal processing is conducted as the cantilever (movable electrode) 13is displaced owing to an acceleration. In addition, by the n⁺ diffusedlayers 4 (see FIG. 2) disposed on the cantilevers 13 (movableelectrodes) and fixed electrodes 14, 15, 16, and 17, the electricpotential is maintained at a constant value.

Although a capacity type acceleration sensor is made in the presentembodiment, if a piezo resistance layer is formed at the surface of theroot portion of the cantilever 13, a piezo resistance type accelerationsensor can be formed. As a matter of course, if these two types ofsensors are formed in a one and same substrate, the precision andreliability of the acceleration sensor can further be improved.

In the acceleration sensor thus produced, the single crystal siliconsubstrate 1 is bonded through an SiO₂ film to the single crystal siliconsubstrate 8, so as to form an SOI structure. In addition, in thecantilever 13, its thickness L2 in a direction parallel to the surfaceof the single crystal silicon substrate 1 is smaller than the thicknessL1 in a direction of the depth of the single crystal silicon substrate1. Accordingly, the cantilever 13 becomes movable, on the surface of thesingle crystal silicon substrate 1, in a direction parallel to thesurface, whereby an acceleration to a direction parallel to thesubstrate surface is detected.

As mentioned in the foregoing, in the present embodiment, on the mainsurface of the single crystal silicon surface 1, there is formed atrench (groove) 3 of a predetermined depth for formation of thecantilever 13 (the first step), and the polysilicon film 6 is formed onthe main surface of the single crystal silicon substrate 1, so as tofill the trench 3 with said polysilicon film 6, and the surface of thepolysilicon film 6 is flattened (smoothed) (the second step).Subsequently, the main surface of the single crystal silicon substrate 1is bonded to the single crystal silicon substrate 8 with an SiO₂ film 9(insulating film) formed thereon, through said SiO₂ film 9 beinginterposed between the substrates 1 and 8 (the third step), whereafterthe reverse side of the single crystal silicon substrate 1 is polishedto a predetermined degree, so as to make the single crystalline siliconsubstrate into a thin film (the fourth step). Subsequently, thesignal-processing circuit 10 is formed on the surface of the singlecrystal silicon substrate, whereafter the polysilicon film 6 is removedby etching from the reverse side of the single crystal siliconsubstrate, so as to form the cantilever 13 (the fifth step).

Accordingly, during the process for the formation of thesignal-processing circuit 10, in the course of the wafer process, thetrench 3 in the surface portion of the single crystal silicon substrate,is filled with the polysilicon film 6, whereby contamination of the ICelements, contamination of production equipment, and degradation ordeterioration of electrical properties accompanied therewith can beprevented. That is, in the wafer process, by contriving to prevent thesurface structures such as concave portions or penetration holes fromappearing on the wafer surface in the heat treatment, photolithographictreatment and the like, in the course of the process, it is possible toprevent contamination and the like, and to thereby stably provideacceleration sensors of high precision.

The thus produced acceleration sensor comprises the single crystalsilicon substrate 1, which is bonded through an SiO₂ film (insulatingfilm) to the single crystal silicon substrate 8, and which is made athin film; the cantilever 13, which is formed on said single crystalsilicon substrate 1 and which is movable in a direction parallel to thesurface of the substrate; and the signal-processing circuit 10, which isalso formed on the single crystal silicon substrate 1 and which performssignal processing as the cantilever 13 moves owing to an acceleration.When an acceleration is applied in a direction parallel to the surfaceof the single crystal silicon substrate 1, the cantilever formed on thesingle crystal silicon substrate 1 moves. As the cantilever 13 moves,signal-processing is performed by the signal-processing circuit 10formed on the single crystal silicon substrate 1. In such a way asabove, by a micro-machining technique using single crystal silicon, anacceleration sensor is formed, by the novel structure of which highprecision and high reliability can be realized.

In addition, since the surface of the aforesaid cantilever 13 and thepart of the single crystal silicon substrate 1, opposing said cantilever13, are coated with the SiO₂ film (insulator) 5, electrode short-circuitin the capacity type acceleration sensor can previously be prevented. Inaddition, it suffices if at least either of the surface of thecantilever 13 or the part of the single crystal silicon substrate 1opposing the cantilever 13 is coated with the SiO₂ film (insulator) 5.

Further, as an application of the present embodiment, as illustrated inFIGS. 11 and 12, the cantilever may be separated from thesignal-processing circuit (IC circuit portion) 10 and an air bridgewiring is formed, in order to reduce parasitic capacity. In addition,the fixed electrodes 14, 15, 16, and 17 may be formed so as to have thesame structure as above. This can be realized by forming a p⁺ typepolysilicon film 7 at the minimum position necessary for bonding thefixed electrodes to the lower substrate.

In addition, although an aluminium wiring layer is used in the aforesaidembodiment, the wiring part may be formed by use of a polysilicon layer.Further, although two movable electrodes are formed at the end of thebeam and simultaneously, four fixed electrodes 14, 15, 16, and 17 areformed, in the aforesaid embodiment, the movable and fixed electrodesmay be formed like the teeth of a comb in order to further improve thesensitivity of the sensor.

In addition, an oxide film may selectively be formed, instead of theformation of the p⁺ polysilicon film 7.

Second Embodiment

Next, there will be made explanations about the production process ofthe second embodiment, emphasizing the points that are different fromthose in the first embodiment. In addition, in the second embodiment tobe hereafter explained, explanations will be made by way of an exampleof a case where a sensor having a structure according to the structureillustrated in FIGS. 1 and 2, as explained in the aforesaid firstembodiment, and there will be illustrated a sectional view correspondingto the 2—2 section of FIG. 1.

In the aforesaid first embodiment, in order to form the cantilever 13,the p⁺ diffused layer (p⁺ polysilicon film) 7 is formed for the purposeof separating the cantilever portion from the single crystal siliconsubstrate at a predetermined distance, but in second present embodiment,a concave portion is formed before formation of a trench, for thepurpose of separating the cantilever from the substrate at apredetermined distance.

In FIGS. 13 to 21, the production process is illustrated.

First, as illustrated in FIG. 13, an n type (100) single crystal siliconsubstrate 20 is provided, and on the main surface of the provided singlecrystal silicon substrate 20, there is formed a concave portion 21 witha predetermined depth e.g. ranging from 0.1 to 5 μm. Subsequently, asillustrated in FIG. 14, on the main surface of the single crystalsilicon substrate 20, there is formed an SiO₂ film 22, and a pattern isformed by a photolithographic means, in the same way as in the aforesaidfirst embodiment. Subsequently, on the main surface of the singlecrystal silicon substrate 20 including the bottom of the concave portion21, there is formed a trench with a depth ranging from about 0.1 toabout 30 μm (3 μm in the present embodiment) by dry etching or the like.

Subsequently, as illustrated in FIG. 15, on the main surface of thesingle crystal silicon substrate 20 including the internal wall of thetrench 23, there is formed an n⁺ diffused layer 24, and an SiO₂ film 25is formed by thermal oxidation. Thereafter, as illustrated in FIG. 16, apolysilicon film 26 is deposited in the trench 23 by the LPCVD method.

Subsequently, as illustrated in FIG. 17, the surface of the polysiliconfilm 26 is polished by use of the SiO₂ film as an etching stopper, so asto smooth the surface. In the above case, although it is desirable thatthe surfaces of the polysilicon film 26 and the SiO₂ film 25 becomesmooth, even if the polysilicon film 26 is rather indented, so long asthe surface of the SiO₂ film is made smooth, no inconvenience is causedin the subsequent wafer cementing.

On the other hand, as illustrated in FIG. 18, another (100) singlecrystal silicon substrate 27 is provided, and, on the main surface ofthe substrate 27, there is formed an SiO₂ film with a thickness rangingfrom 0.1 to 1.0 μm by thermal oxidation of the substrate. Subsequently,single crystal silicon substrates 20 and 27 are dipped in e.g. asolution of an aqueous hydrogen per oxide and sulfuric acid, so as tosubject them to a hydrophilicity-imparting treatment. Subsequently, thesubstrates are dried, and thereafter, the main surfaces of the twosingle crystal silicon substrates 20 and 27 are put together at roomtemperature, and introduced into a furnace at 400 to 1100° C. for aperiod of time ranging from 0.5 and 2 hours, so as to strongly bond thetwo surfaces.

Subsequently, as illustrated in FIG. 19, the reverse side of the singlecrystal silicon substrate 20 is subjected to selective polishing by useof an aqueous solution of alkali type, e.g. an aqueous KOH solution. Theselective polishing is performed until the SiO₂ film 25 appears on thesurface. As a result, the thickness of the single crystal siliconsubstrate 20 becomes e.g. about 3 μm, so as to be made a thin film.

Subsequently, as illustrated in FIG. 20, a signal-processing circuit (ICcircuit portion) 10 is formed through an ordinary CMOS process, bipolarprocess, or the like. Further, on the upper surface of thesignal-processing circuit 10, there is formed, as a passivation film 11,a plasma SiN film (P-SiN film) by e.g. plasma CVD method. Subsequently,windows 12 are opened at predetermined areas of the passivation film 11,and the polysilicon film 20 is exposed to the surface at the sensorportion.

Subsequently, as illustrated in FIG. 21, by use of a 20% solution ofTMAH (tetramethylammonium hydroxide) (CH₃)₄NOH, the polysilicon film 26is removed by etching from the reverse side of the single crystalsilicon substrate 20 through the windows 12 on the passivation film 11.In the above case, the passivation film 11 (P-SiN), SiO₂ film, andaluminium wiring layer are hardly etched by the selective etching.

As a result, a cantilever 13 is formed.

Also by the present embodiment, there is obtained the same effect as inthe aforesaid first embodiment.

Third Embodiment

Next, there will be made explanations about the production process inthe third embodiment, laying stress on the differential points betweenthe first and third embodiments.

Although, in the aforesaid first and second embodiments, the trench isfilled with polysilicon before the bonding of the wafers, in the presentembodiment, the trench is filled with polysilicon after the bonding ofwafers, and in the final stage, the thus filled polysilicon is removed,so as to produce an acceleration sensor.

In FIGS. 22 to 28, the production process is illustrated.

First, as illustrated in FIG. 22, an n-type (100) single crystal siliconsubstrate 30 is provided, and on the main surface of the provided singlecrystal silicon substrate 30, there is formed a concave portion 31 in adepth ranging from 0.1 to 5 μm, in the same way as in the aforesaidsecond embodiment. On the other hand, as illustrated in FIG. 23, asingle crystal silicon substrate 32 is provided, and an SiO₂ film isformed by thermal oxidation on the main surface of the single crystalsilicon substrate 31. Thereafter, the main surface of the single crystalsilicon substrate 30 is bonded to the main surface of a single crystalsilicon substrate 32.

Subsequently, as illustrated in FIG. 24, the reverse side of the singlecrystal silicon substrate 30 is subjected to mirror polishing to apredetermined thickness (0.1 to 30 μm). Thereafter, as illustrated inFIG. 25, there is formed an SiO₂ film 34 to a thickness ranging from 0.1to 2 μm, following which the SiO₂ film is subjected to patterning, and atrench 35 is formed by etching. Thereby, a cantilever 13 and atransversal insulatedly separated area of the processing circuit portionare formed.

Next, by thermal diffusion or the like, there is introduced an N typeimpurity of arsenic or phosphorus in a high concentration, and a highlyconcentrated n⁺ layer 36 is formed in the silicon area which is notcovered with SiO₂ films 33 and 34.

Subsequently, as illustrated in FIG. 26, a thermal oxidation film isformed on the side wall of the trench 35 and the like, whereafter apolysilicon film 37 is formed on the surface of the single crystalsilicon substrate 30, and the trench 35 is filled with the polysiliconfilm 37. Thereafter, as illustrated in FIG. 27, the surface of thepolysilicon film 37 is selectively polished and smoothed until the SiO₂film 34 appears on the surface. Further, as illustrated in FIG. 28, asignal-processing circuit 10 is formed, and finally, the polysiliconfilm 37 is removed by etching from the reverse side (upper surface side)of the single crystal silicon substrate 30, so as to again separate thecantilever 13 from the substrate to allow it to move.

As described above, in the present third embodiment, the main surface ofthe single crystal silicon substrate 30 is bonded to the single crystalsilicon substrate 32 with the SiO₂ film (insulating film) 33 formedthereon, through said SiO₂ film 33 being interposed between thesubstrates 30 and 32 (first step), and the reverse side of the singlecrystal silicon substrate is polished to a predetermined degree, so asto make the single crystal silicon substrate 30 a thin film (secondstep). Subsequently, on the reverse side of the single crystal siliconsubstrate 30, there is formed a trench (groove) 35 with a predetermineddepth for formation of a cantilever 13 (third step), and the trench 35is filled with the polysilicon film 37, and the surface of thepolysilicon film 37 is smoothed (fourth step). Subsequently, asignal-processing circuit is formed on the single crystal siliconsubstrate 30, whereafter the polysilicon film 37 is removed, by etching,from the reverse side of the single crystal silicon substrate 30, so asto form a cantilever 13 (fifth step).

Therefore, in the process of formation of the signal-processing circuit10 in the wafer, the trench 35 is filled with the polysilicon film 37 inthe upper surface part of the single crystal silicon substrate 30,whereby contamination of the IC elements, contamination of theproduction apparatus, and degradation or deterioration of electricalproperties attended therewith can be prevented. That is, by contrivingnot to make a surface structure such as a concave portion or aperforation of the like appear on the wafer surface in the heattreatment, the phololithographical treatment or the like in the courseof the wafer process, contamination and the like can be prevented, so asto stabilize the wafer process, and a stable supply of accelerationsensors, of high precision, can be produced.

Fourth Embodiment

In the following, there will be made explanations about the productionprocess of the fourth embodiment, laying stress on the differencesbetween the present embodiment and the third embodiment.

The present embodiment is intended to produce an acceleration sensor ata lower cost than the production process of the third embodiment.

In FIGS. 29 to 31, the production process is illustrated.

First, as illustrated in FIG. 29, an SiO₂ film with a thickness rangingfrom 0.1 to 2 μm is formed on the main surface of a single crystalsilicon substrate 40, and a single crystal silicon substrate 42 isbonded to the substrate 40 so as to interpose the SiO₂ film 41 betweenthe substrates 40 and 42. Subsequently, as illustrated in FIG. 30, theupper surface of the single crystal silicon substrate 42 is polished soas to make the thickness of the single crystal silicon substrate apredetermined value. That is, the thickness of the single crystalsilicon substrate 42 is reduced to e.g. about 3 μm. Thereafter, a highlyconcentrated n⁺ diffused layer 43 is formed on the upper surface of thesingle crystal silicon substrate 42, and further, an SiO₂ film 44 isformed thereon. It suffices if this highly concentrated n⁺ diffusedlayer 43 is formed corresponding to the parts of the sensor which willbe formed as movable electrodes and fixed electrodes.

Subsequently, as illustrated in FIG. 31, patterning is applied to anSiO₂ film, as in the third embodiment, and a trench 45 is formed on thesingle crystal silicon substrate 42. Thereafter, the SiO₂ film 41 belowthis trench 45 is partly removed by etching with an aqueous hydrofluoricacid solution. In this case, a part of SiO₂ film 41, below the part tobe formed as a cantilever 13, is completely removed, and another part ofthe SiO₂ film 41, below the part to be formed as solid electrodes andsignal-processing circuit portion, is left. Because the lower portion ofthe part is to be formed as the cantilever 13 it is narrower than withthe other part. In addition, in the above case, the highly concentratedn⁺ diffused layer 43 is separated into the cantilever 13 and fixedelectrodes.

Subsequently, through the same processes as illustrated in FIGS. 26 to28, there is produced a capacity type acceleration sensor.

In the following, there will be explained application examples of thefourth embodiment with reference to FIGS. 32 to 34.

First, as illustrated in FIG. 32, an SiO₂ film with a thickness rangingfrom 0.1 to 2 μm is formed on the main surface of the single crystalsilicon substrate 40, and a concave portion 47 with a depth ranging from0.1 to 3 μm is formed at the area of the main surface of the singlecrystal silicon substrate 42, on which area a cantilever is to beformed. Subsequently, the main surface of the single crystal siliconsubstrate 42 is bonded to the main surface of the single crystal siliconsubstrate 40, the SiO₂ film 41 being put therebetween. Further, asillustrated in FIG. 33, the upper surface of the single crystal siliconsubstrate 42 is polished so as to give the single crystal siliconsubstrate 42 a predetermined thickness. That is, the thickness of thesingle crystal silicon substrate 42 is made thin to a value of about 3μm. Subsequently, the aforesaid highly concentrated diffused layer 43 isformed on the upper surface of the single crystal silicon substrate 42,and an SiO₂ film 44 is formed thereon.

Subsequently, as illustrated in FIG. 34, onto the single crystal siliconsubstrate 42, there are formed trenches 45 extending to the concaveportions 47, and cantilevers 13 are formed.

Thereafter, a capacity type acceleration sensor is produced through thesame processes as illustrated in FIGS. 26 to 28.

By conducting the aforesaid procedures, electrical insulation can bemade more securely as compared with the case where the SiO₂ film 41 ispartly removed by etching. In addition, the mechanical strength of thesensor can be improved.

In addition, the present invention is not restricted to the aforesaidembodiments, and is applicable to a twin-lever spring or a polyeverspring embodiment, in addition to a cantilever spring embodiment.

In addition, as illustrated in FIG. 35, it is possible to form twoacceleration sensors 13a and 13b onto a single crystal silicon substrate50, for detecting the acceleration in the direction X by theacceleration sensor 13a, and the acceleration in the direction Y by theacceleration sensor 13b. Further, it is possible to form an accelerationsensor capable of detecting the acceleration in a direction surfaceperpendicular to these X and Y direction acceleration sensors 13a and13b, an the same substrate, so as to detect an acceleration inthree-dimensionals. In addition, when the present acceleration sensor isused as a capacity type acceleration sensor, it is possible to furtherstabilize the properties by forming the present acceleration sensor intoa so-called “servo type” sensor (with a closed-loop circuit).

In addition, in the aforesaid embodiments, the trenches (grooves) 3, 23,and 35 are filled with polysilicon films 6, 26, and 37, respectively,but there may be used a film of polysilicon, amorphous silicon or amixed silicon containing polysilicon and amorphous silicon.

In addition, in the aforesaid embodiments, a sensor portion and asignal-processing portion are formed in the single crystal siliconsubstrate to be formed as the upper side, but the present accelerationsensor is not restricted to such a structure, and it is possible toutilize also a single crystal silicon substrate formed as a base, andform a sensor portion and a signal-processing portion on the lowersubstrate.

As described in detail in the foregoing, according to the presentinvention, high precision and high reliability can be realized byformatting a novel structure. In addition, when a signal-processingcircuit is provided on the same chip as a movable beam, since there isproduced neither a hollow part nor a groove during the productionprocess, it is possible to make the processing stable. In addition, itis at the final step that the movable beam is made movable with respectto the substrate, whereby, in the case of the movable beam being bondedto the lower substrate to be formed as a pedestal or in the case of acircuit being formed, or the like, minute beams can be prevented frombeing broken, and the yield thereof can be made higher. In addition,since it is a micro-machining technique which determines the shape ofthe present acceleration sensor, the present acceleration sensor can beproduced with high precision.

[Industrial Availability]

As described in the foregoing, the present invention is useful for theproduction of a semi-conductor acceleration sensor having minute movableparts, and the present acceleration sensor is suitable as anacceleration sensor to be used for air bag system, suspension controlsystem and the like of automobiles. In addition, the present inventioncan be a applied to a capacity type acceleration sensor for detectingacceleration in multiple directions.

1. An acceleration sensor comprising: a first substrate formed of asilicon material which is used as a conductive material; a secondsubstrate provided on the lower side of said first substrate andelectrically insulated from the first substrate; said first substrateincluding: a support beam having a mass portion forming capacitiveelectrodes for displacement in a parallel direction to a surface of saidsecond substrate according to the degree of acceleration, a fixedportion for fixing said support beam to said second substrate and asupport portion for intermediately supporting said mass portion to saidfixed portion, an insulating groove extending through a thickness ofsaid first substrate around the entire periphery of said support beam,and stationary blocks forming capacitive electrodes defined by saidinsulating groove on the outer sides of said support beam separatelyacross said insulating groove and fixed to said second substrate; andgap means forming a gap space in order to space said mass portion andsaid supporting portion from a surface of said second substrate; andsaid second substrate being separated from said first substrate by aninsulating layer which is at least provided on the lower side of saidfixed portion and stationary blocks.
 2. An acceleration sensor asdefined in claim 1, wherein said insulating groove defines a narrowdetection groove between said mass portion of said support beam and eachof said stationary blocks, and movable and stationary electrodes areformed on lateral side surfaces of said support beam and said stationaryblocks in face to face relation with each other across said detectiongroove.
 3. An acceleration sensor as defined in claim 1, wherein saidsupport beam is securely fixed to said second substrate at said fixedend, and reduced in a width in said support portion to provide at leastone support for said mass portion in a fore free end portion to bedisplayed horizontally according to the degree of acceleration in thefashion of at least one fulcrum point type acceleration sensor.
 4. Anacceleration sensor as defined in claim 3, wherein said width of saidsupport portion is smaller than a thickness of said fixed portion andsaid support beam provides a cantilever type support.
 5. An accelerationsensor as defined in claim 1, wherein said first substrate is formed ofsilicon material with a (100) crystal face.
 6. An acceleration sensor asdefined in claim 5, wherein said first substrate is formed of a n-typesilicon material.
 7. An acceleration sensor as defined in claim 1,wherein said second substrate is formed of a silicon material having asurface covered with an insulating oxidation film.
 8. An accelerationsensor comprising: a substrate which is selected from the groupconsisting of an insulating material and oxidized semiconductormaterial; a support beam which includes a mass portion forming apredetermined mass and first capacitive electrodes on side surfaces ofsaid mass portion, a fixed portion for fixing said support beam to saidsubstrate and a thin support portion for intermediately connectingbetween said mass portion and said fixed portion; a pair of stationaryblocks arranged on both sides of said support beam separately across anair gap and fixed to said substrate, said stationary blocks providedwith second capacitive electrodes on the opposite sides of firstcapacitive electrodes of said mass portion; gap means forming a gapspace in order to space said mass portion and thin support portion froma surface of said substrate; and said mass portion being displace in aparallel direction to the surface of said substrate according to thedegree of acceleration and said support beam and stationary blocksformed of a silicon material which is used as a conductive material andelectrically insulated from said substrate.
 9. An acceleration sensor asdefined in claim 8, wherein said gap means is formed of an insulatinglayer at least between said substrate and said fixed portion of thesupport beam and said stationary blocks.
 10. An acceleration sensor asdefined in claim 8, wherein movable and stationary electrodes are formedon lateral side surfaces of said mass portion and said stationary blocksin face to face relation with each other across said air gap.
 11. Anacceleration sensor as defined in claim 8, wherein a width of said thinsupport portion is smaller than a thickness of said fixed portion. 12.An acceleration sensor as defined in claim 8, wherein said support beamand stationary blocks are formed of a n-type silicon material with a(100) crystal face.
 13. An acceleration sensor as defined in claim 8,wherein said substrate is formed of a silicon material having a surfacecovered with an insulating oxidation film.
 14. An acceleration sensorcomprising: A. a first single crystalline silicon substrate having afirst surface and a second surface opposite said first surface; B. asecond single crystalline silicon substrate connected to a side of saidfirst surface of said first single crystalline silicon substrate with aninsulating layer interposed therebetween; C. said first singlecrystalline silicon substrate including; i. a movable beam defined by atrench which is disposed to surround said movable beam and extend fromsaid second surface to said first surface, said movable beam beingsupported by said second single crystalline silicon substrate throughsaid insulating layer to be displaceable in a direction parallel to saidfirst surface of said first single crystalline silicon substrate, ii. astationary block disposed to be spaced apart from said movable beam viasaid trench, facing said movable beam to form a pair of capacitiveelectrodes with said movable beam, and fixed to said second singlecrystalline silicon substrate; and D. a signal-processing circuitelement for carrying out a processing operation based on a change of acapacitance between said capacitive electrodes.
 15. An accelerationsensor according to claim 14, wherein said movable beam has a firstthickness along said direction parallel to said first surface of saidfirst single crystal silicon substrate and a second thickness in adirection perpendicular to said first surface of said first crystalsilicon substrate, said first thickness being smaller than said secondthickness.
 16. An acceleration sensor according to claim 14, wherein atleast one of a surface of said movable beam exposed to said trench and asurface of said first single crystal silicon substrate facing saidmovable beam is covered with an insulator.
 17. An acceleration sensoraccording to claim 14, wherein said movable beam or said stationaryblock comprise an impurity-doped region to form said capacitiveelectrodes.
 18. An acceleration sensor according to claim 14, furthercomprising first, second and third stationary blocks, each of which isformed of a part of said first single crystal silicon substrate, andwherein said movable beam includes a first branch and a second branch,said first branch being interposed between said stationary block andsaid first stationary block with a first air gap defined therebetween,said second branch of said movable beam being interposed between saidsecond and said third stationary blocks with a second air gap definedtherebetween, said stationary block and said second stationary blockbeing electrically connected with each other, said first and said thirdstationary blocks being electrically connected with each other.
 19. Anacceleration sensor according to claim 14, wherein saidsignal-processing circuit is disposed on said first single crystalsilicon substrate.
 20. An acceleration sensor comprising: A. a firstsingle crystalline silicon substrate having a first surface and a secondsurface opposite said first surface; B. a second single crystallinesilicon substrate connected to a side of said first surface of saidfirst single crystalline silicon substrate with an insulating layerinterposed therebetween; C. said first single crystalline siliconsubstrate being divided by a trench extending from said second surfaceto said first surface; i. a movable beam portion surrounded by saidtrench, said movable beam portion being supported by said second singlecrystalline silicon substrate through said insulating layer to bedisplaceable in a direction parallel to said first surface of said firstsingle crystalline silicon substrate, said movable beam portion having amovable electrode, ii. a stationary portion disposed to be spaced apartfrom said movable beam via said trench, having a stationary electrodewhich faces said movable beam via said trench to form a pair ofcapacitive electrodes with said movable electrode, and fixed to saidsecond single crystalline silicon substrate; and D. an insulatorcovering at least one of a surface of said movable electrode and asurface of said stationary electrode.
 21. An acceleration sensoraccording to claim 20, further comprising a signal-processing circuitdefined in the first single crystal silicon substrate for carrying out aprocessing operation based on a change of a capacitance between saidmovable electrode and said stationary electrode.
 22. A semiconductordynamic amount sensor comprising a movable portion and a stationaryportion located on a side of a support, the movable portion and thestationary portion being formed of the same semiconductor material, saidsupport being present on an insulating layer formed on a surface of asubstrate, said movable portion and said stationary portion and saidsubstrate being electrically separated through said insulating layer,wherein at least one of the facing surfaces of said movable portion andsaid stationary portion facing with each other is provided with aprotrusion which protrudes toward the opposite-facing surface thereof ina direction parallel with the surface of the substrate and shortens agap between said movable portion and said stationary portion there,wherein said movable portion can be moved in a direction parallel withsaid surface of said substrate.
 23. The semiconductor dynamic amountsensor according to claim 22, wherein said movable portion comprises asupport portion and a displacing portion connected through a beamstructure to said support portion and having said facing surface facingsaid stationary portion; said facing surfaces of said movable portionand said stationary portion facing with each other are respectivelyprovided with a stationary electrode and a movable electrode; and saiddisplacing portion is displaced upon an external force applied to saiddisplacing portion in such a manner that said movable portion and saidstationary portion come close and apart.
 24. The semiconductor dynamicamount sensor according to claim 23, wherein said stationary electrodeor said movable electrode is an n⁺-type semiconductor region.
 25. Thesemiconductor dynamic amount sensor according to claim 24, wherein atleast one of said stationary electrode or said movable electrode iscovered with an insulating layer.
 26. The semiconductor dynamic amountsensor according to claim 23, wherein said semiconductor materialcomposing said stationary electrode and said movable electrode is asingle crystal semiconductor.
 27. The semiconductor dynamic amountsensor according to claim 23, which is a capacitor-type dynamic amountsensor, in which a signal generated is according to a variation ofcapacitor between said movable electrode and said stationary electrodevaried upon displacement of said displacing portion.
 28. A semiconductordynamic amount sensor comprising: a substrate; an insulating layerformed on a surface of said substrate; a support made of an insulatingsubstrate and formed on said insulating layer; a movable portion and astationary portion provided on a side of said support, each being formedof the same semiconductor substrate by etching and being separated witheach other in a direction parallel with a surface of said support, saidmovable portion and said stationary portion and said substrate beingelectrically separated through said insulating layer, said movableportion being able to be moved in a direction parallel with said surfaceof said substrate; and a protrusion provided with at least one of thefacing surfaces of said movable portion and said stationary portionfacing with each other, protruding toward the opposite-facing surfacethereof in the direction parallel with said surface of said substrate,and ensuring a minimum gap between said facing surfaces even when saidfacing surfaces come close.
 29. The semiconductor dynamic amount sensoraccording to claim 28, wherein said movable portion comprises a supportportion and a displacing portion connected through a beam structure tosaid support portion and having said facing surface facing saidstationary portion; said facing surfaces of said movable portion andsaid stationary portion facing with each other are respectively providedwith a stationary electrode and a movable electrode; and said displacingportion is displaced upon an external force applied to said displacingportion in such a manner that said movable portion and said stationaryportion come close and apart.
 30. The semiconductor dynamic amountsensor according to claim 29, wherein said stationary electrode or saidmovable electrode is an n⁺-type semiconductor region.
 31. Thesemiconductor dynamic amount sensor according to claim 30, wherein atleast one of said stationary electrode or said movable electrode iscovered with an insulating layer.
 32. The semiconductor dynamic amountsensor according to claim 29, wherein said semiconductor materialcomposing said stationary electrode and said movable electrode is asingle crystal semiconductor.
 33. The semiconductor dynamic amountsensor according to claim 29, which is a capacitor-type dynamic amountsensor, in which a signal generated is according to a variation ofcapacitor between said movable electrode and said stationary electrodevaried upon displacement of said displacing portion.
 34. A semiconductordynamic amount sensor comprising a movable portion and a stationaryportion located on a side of a support and each formed of the samesemiconductor material, said support being present on an insulatinglayer formed on a surface of a substrate said movable portion and saidstationary portion and said substrate being electrically separatedthrough said insulating layer, wherein the facing surfaces of saidstationary portion and said movable portion facing each other areprovided with a stationary electrode and a movable electroderespectively, said movable portion being able to be moved in a directionparallel with said surface of said substrate and a portion or portionsof said facing surfaces of said stationary portion and said movableportion with said stationary electrode and said movable electrodeprotrudes or protrude in a direction in parallel with the surface ofsaid support such that said stationary portion and said movable portionhave a first gap and a second gap smaller than said first gap betweensaid facing surfaces.
 35. The semiconductor dynamic amount sensoraccording to claim 22, wherein said surfaces of said stationary portionand said movable portion are present in a plurality of number.
 36. Thesemiconductor dynamic amount sensor according to claim 28, wherein saidsurfaces of said stationary portion and said movable portion are presentin a plurality of number.
 37. The semiconductor dynamic amount sensoraccording to claim 34, wherein said surfaces of said stationary portionand said movable portion are present in a plurality of number.
 38. Thesemiconductor dynamic amount sensor according to claim 22, wherein saidstationary portion and said movable portion are in the form of teeth ofa comb.
 39. The semiconductor dynamic amount sensor according to claim28, wherein said stationary portion and said movable portion are in theform of teeth of a comb.
 40. The semiconductor dynamic amount sensoraccording to claim 34, wherein said stationary portion and said movableportion are in the form of teeth of a comb.